This invention relates to a process for the preparation of quinacridone pigments having reduced particle size and improved coloristic properties. In particular, the particle size of the typical unsubstituted quinacridone is reduced by about 30% by the addition of certain salts, preferably metal salts, to the ring closure melt during pigment synthesis and before the drowning process. The resultant quinacridones have characteristically deeper masstone, increased transparency, and generally bluer undertone (tint) hue.
Processes for the preparation of quinacridones are well known and documented. For example, S. S. Labana and L. L. Labana, "Quinacridones," Chemical Reviews, 67, 1-18 (1967), and U.S. Pat. Nos. 3,157,659, 3,256,285, and 3,317,539. The quinacridones thus obtained, known as crude quinacridones, are generally unsuitable for use as pigments and must undergo one or more additional finishing steps to modify the particle size, particle shape, or crystal structure to achieve pigmentary quality.
A preferred method for preparing quinacridones involves thermally inducing ring closure of 2,5-dianilinoterephthalic acid intermediates, as well as known aniline-substituted derivatives thereof, in the presence of polyphosphoric acid, such as described in U.S. Pat. No. 3,257,405. After ring closure is completed, the melt is drowned by pouring into a liquid in which the quinacridone is substantially insoluble, usually water or an alcohol. The resultant crystalline pigment that is then further conditioned by solvent treatment or milling in combination with solvent treatment.
Final particle size of quinacridone pigments is controlled by the methods used both in synthesis and in aftertreatment. Quinacridone pigments can be made more transparent by reducing the particle size or more opaque by increasing the particle size. Particle size is most often controlled during precipitation of the pigment by drowning or during subsequent milling of the crude pigment. Tinctorial strength and transparency of pigments can also be affected by solvent treatment.
Quinacridones and quinacridone solid solutions disclosed in the prior art are also sensitive to numerous other process variables, such as the type of drowning solvent, the temperature of the drowning solvent, the type of agitation used during the drowning process, the duration of the drowning process, and the post-drown processes. These variables are known to affect crystal phase, particle size and distribution, and surface characteristics, all of which ultimately affect the pigment color properties such as transparency, hue, tinctorial strength, dispersibility, dispersion stability, and rheology.
It is possible to obtain quinacridone pigments having smaller particle size by adding iron salts during the ring closure reaction beyond the amount of iron that is commonly introduced in small quantities during commercial preparation of the 2,5-dianilinoterephthalic acid precursor. E.g., copending application Ser. No. 08/239.180 (filed May 6, 1994). However, it has now been found that quinacridone pigments having smaller particle size can be obtained without the need for milling processes or the narrow, low-temperature ranges typically required in known processes by adding salts other than iron salts during the preparation of the pigments, particularly during the ring closure reaction. The process of the present invention is in principle applicable to all quinacridone pigment manufacturing processes that include an "acid pasting" step, but the greatest improvement in coloristic properties is observed with ring-closure processes, including processes used to prepare quinacridone solid solutions.